Two terms that are commonly used by statisticians in the healthcare and epidemiology industries are **incidence rate** and **cumulative incidence**.

These terms provide us with slightly different information and are both useful to know when studying a disease.

The **incidence rate** refers to the rate that a disease occurs in a population relative to the total observation time that individuals are at risk.

It is calculated as:

**Incidence rate**= # New cases of disease during observation period / Total observation time at risk

The **cumulative incidence** refers to the portion of people in a population who develop a disease during a specific observation time period relative to the total number of people at risk at the beginning of an observation time period.

It is calculated as:

**Cumulative incidence**= # people at risk who develop the disease during observation period / Total people at risk at beginning of observation time period

The following example shows how to calculate both of these metrics in practice.

**Example: How to Calculate Incidence Rate & Cumulative Incidence**

Suppose a statistician is studying a particular disease in a population. For simplicity, we’ll refer to it as disease A.

Suppose the statistician wants to calculate both incidence rate and cumulative incidence to understand the rate that this disease occurs in a population along with the portion of individuals who can be expected to catch the disease.

Assume that statistician completes a study with the following data:

- Time period:
**1 year** - Number of individuals at beginning of study:
**10,000** - Number of new cases of disease during study:
**400**

The statistician can use the following formula to calculate the incidence rate:

**Incidence rate**= # New cases of disease during observation period / Total observation time at risk- Incidence rate = 400 / 10,000 * 1 year
- Incidence rate = 400 / 10,000 person-years

We would say that the incidence rate is** 400 new cases per 10,000 person-years**.

If we’d like, we can also divided by 10,000 to express this number in terms of 1 person-years.

For example, this is the same as .04 new cases per 1 person-years.

The statistician can use the following formula to calculate the cumulative incidence:

**Cumulative incidence**= # people at risk who develop the disease during observation period / Total people at risk at beginning of observation time period- Cumulative incidence = 400 / 10,000
- Cumulative incidence = .04

We would say that the cumulative incidence of this particular disease is **.04**. In simple terms, this helps us understand that 4% of the total population at risk caught the disease during the 1-year time frame of the study.

**When to Use Cumulative Incidence vs. Incidence Rate**

In most studies, it’s useful to calculate both the cumulative incidence and the incidence rate since both metrics provide us with different information.

The **cumulative incidence** gives us an idea of the portion of individuals who catch a disease, which will always range between 0% and 100%.

Conversely, the **incidence rate** gives us an idea of how often the disease occurs using person-years as the denominator.

By using both of these metrics, a statistician can summarize the rate at which a particular disease occurs.

**Additional Resources**

The following tutorials explain how to calculate other common metrics in statistics:

What is Prevalence in Statistics?

What is Number Needed to Harm?

Number Needed to Harm Calculator